Kinetics of receptor occupation and anticonvulsive effects of diazepam in rats

Time courses of anticonvulsive effect nd benzodiazepine receptor occupation in the brain were determined after iv administration of diazepam (DZP) (1.2 mg/kg) to rats. Score for the anticonvulsive effect was assigned to each DZP-treated rat depending on the degree of protection against pentylenetetrazole-induced seizures. Measurement of in vivo receptor occupation was made by comparing the specific 3H-DZP binding in vitro to the crude synaptosomal fractions between vehicle-treated rats (control) and DZP-injected rats. The receptor occupation declined in parallel with the anticonvulsive effect with approximate half-life of 1 hr. A good linear correlation (r = 0.977) was observed between the receptor occupation and the anticonvulsive effect. The time course of anticonvulsive effect was further compared with that of DZP concentration in the brain which was previously reported [Igari et al., Drug Metab. Dispos. 10, 676 (1982)]. The anticonvulsive effect was nonlinearly related to the DZP concentration in the brain with the half-maximum concentration, Kd in vivo of 793 nM. The half-maximum concentration for the unbound DZP, Kd in vivo was also estimated to be 113 nM, which was still 50 times the values of Kd (2.3 nM) determined at 0 degree C from an in vitro binding study of 3H-DZP to the receptor in the crude synaptosomal fraction.